Gas streams containing condensable components, such as sulfur dioxide or various organic vapors, arise from numerous industrial and commercial processes. Venting such gases to the atmosphere wastes resources and causes pollution problems. Industries throughout the world are, therefore, under increasing pressure to clean up waste gas emissions. A widely used treatment method is condensation. The idea is to cool and/or compress the gas beyond the dewpoint of the condensable constituent. A portion of the condensable component will then condense out and can be drawn off in liquid form for reuse or disposal. The degree of removal that can be achieved in this way will depend on the initial concentration, the boiling point of the condensable, and the operating conditions of the process. Problems encountered in such processes are 1) low concentration of the condensable component in the stream, and/or low boiling point, so that the dew point is difficult to reach, and 2) need for regular defrosting. Compressing the gas stream above about 10-15 atmospheres requires large energy consumption and costs increase rapidly in proportion to compressor capacity. If the gas has to be cooled below 0.degree. C., then ice formation in the condenser from water vapor entrained in the feed vapor may occur. Even if the gas stream is pre-dried, taking it down to cryogenic temperatures will again be a costly, energy intensive procedure. These practical matters tend to limit the extent of condensable removal that can be attained. Even under favorable operating conditions, 20% or more of the condensable component may be left in the non-condensed bleed gas from the condenser.
Cryogenic condensation and compression/condensation units have been in widespread use for many years. Condensation is a valuable method of waste treatment and pollution control. Nevertheless there remains a longstanding need to improve condensation technology. Recent evidence concerning the adverse environmental effects of halogenated hydrocarbons and chlorofluorocarbons (CFCs) has dramatically intensified that need.